Problems associated with known tests stem from the fact that in each case the observer is required to give some sort of indication or response as to when they see the target in their visual field. The exact point of when the target is seen can be somewhat subjective and unclear and can therefore lead to significant inaccuracies.
More recently, tests have been developed to address the problem of subjectiveness associated with the above tests. Notably, WO 99/226338 describes testing apparatus in which an eye tracker is used to ensure the observer maintains their direction of gaze on a fixation target as other targets are presented on screen. This system offers considerably more accurate visual field results since an observer cannot lose their direction of gaze on the fixation target as new targets are presented. However, this system can suffer from a problem in that the eye tracker used to track a patient's direction of gaze, and the associated processors required to operate the eye tracker, are expensive. Furthermore, eye trackers currently available can suffer from inaccuracies, making operation of the system problematic, particularly if not properly configured.